WASHINGTON — The Defense Department has grounded its fleet of F-35 fighter jets after one of them caught fire as it was preparing to take off at Eglin Air Force Base in Florida.

The grounding is the second in a month and the latest in a long string of delays that has plagued the Air Force’s newest fighter aircraft. It comes as the plane was to make its international flying debut during showings to prospective buyers at two air shows in Britain beginning next week.

The Air Force has chosen the Vermont National Guard site at Burlington International Airport in South Burlington as a future base for the F-35s. Opponents are fighting that decision on the grounds of noise and the potential effect on nearby property values, as well as the possibility of a jet crash.

The cause of the problem at Eglin two weeks ago remains under investigation, the Pentagon’s press secretary, Rear Adm. John F. Kirby, said in a statement Thursday night. “Additional inspections of F-35 engines have been ordered, and return to flight will be determined based on inspection results and analysis of engineering data,” he said.

The Pentagon is seeking to determine whether the Eglin fire was an isolated incident or signaled a wider problem in the F-35 fleet, which has variations designed for the Air Force, Navy and Marine Corps.

In June, an oil leak in an F-35 engine was detected during flight, and although the pilot safely landed at Marine Corps Air Station Yuma in Arizona, the Pentagon grounded the fleet for three days.

The F-35s, which are manufactured by Lockheed Martin, are designed to bring stealth ability to the country’s fighter fleet. The plane’s backers say it will also provide a versatility that will allow it to operate in virtually any battle situation.

As the only model available for export from the United States’ so-called fifth generation of fighter aircraft, the F-35 has fetched many orders on the global market. Australia, Britain, Canada, Israel, Italy, Japan, the Netherlands, Norway and Turkey are among the expected buyers.

The planes, however, have run into delays amid criticism of the program’s cost, which has ballooned to about $400 billion, or about $160 million per plane, according to the Government Accountability Office.

Britain has several F-35 jets for testing and training, and it is expected to buy 138 of them.

During the dedication Friday of Britain’s newest aircraft carrier, the Queen Elizabeth, an F-35 was supposed to make a flyover, but it remained on the ground at Eglin.

It is unclear whether the planes will make it to Britain next week for demonstrations at the Royal International Air Tattoo and the Farnborough International Airshow.